Order this book today at
www.elsevierdirect.com or by calling 1-800-545-2522 and receive an additional 20% discount. Use promotion code 92839 when ordering. Offer expires 07/31/2008. Valid only in North America.
Part 2 explains Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM).
Radio signals can be used to carry information. The information, which may be audio, data or other forms, is used to modify (modulate) a single frequency known as the carrier. The information superimposed onto the carrier forms a radio signal which is transmitted to the receiver. Here, the information is removed from the radio signal and reconstituted in its original format in a process known as demodulation. It is worth noting at this stage that the carrier itself does not convey any information.
There are many different varieties of modulation but they all fall into three basic categories, namely amplitude modulation, frequency modulation and phase modulation, although frequency and phase modulation are essentially the same. Each type has its own advantages and disadvantages. A review of all three basic types will be undertaken, although a much greater focus will be placed on those types used within phone systems. By reviewing all the techniques, a greater understanding of the advantages and disadvantages can be gained.
Radio carrier
The basis of any radio signal or transmission is the carrier. This consists of an alternating waveform like that shown in Figure 3-1. This is generated in the transmitter, and if it is radiated in this form it carries no information – it appears at the receiver as a constant signal.
Figure 3-1. An alternating waveform.
Amplitude modulation
Possibly the most obvious method of modulating a carrier is to change its amplitude in line with the modulating signal.
The simplest form of amplitude modulation is to employ a system known as 'on–off keying' (OOK), where the carrier is simply turned on and off. This is a very elementary form of digital modulation and was the method used to carry Morse transmissions, which were widely used especially in the early days of 'wireless'. Here, the length of the on and off periods defined the different characters.
More generally, the amplitude of the overall signal is varied in line with the incoming audio or other modulating signal, as shown in Figure 3-2. Here, the envelope of the carrier can be seen to change in line with the modulating signal. This is known as Amplitude Modulation (AM).
Figure 3-2. An amplitude modulated signal.
The demodulation process for AM where the radio frequency signal is converted into an audio frequency signal is very simple. It only requires a simple diode detector circuit like that shown in Figure 3-3. In this circuit the diode rectifies the signal, only allowing the one-half of the alternating radio frequency waveform through. A capacitor is used as a simple low-pass filter to remove the radio-frequency parts of the signal, leaving the audio waveform. This can be fed into an amplifier, after which it can be used to drive a loudspeaker. This form of demodulator is very cheap and easy to implement, and is still widely used in many AM receivers today.
Figure 3-3. A simple diode detector circuit.
The signal may also be demodulated more efficiently using a system known as synchronous detection (Figure 3-4). Here, the signal is mixed with a locally generated signal with the same frequency and phase as the carrier. In this way the signal is converted down to the baseband frequency. This system has the advantage of a more linear demodulation characteristic than the diode detector, and it is more resilient to various forms of distortion. There are various methods of generating the mix signal. One of the easiest is to take a feed from the signal being received and pass it through a very high-gain amplifier. This removes any modulation, leaving just the carrier with exactly the required frequency and phase. This can be mixed with the incoming signal and the result filtered to recover the original audio.
Figure 3-4. Synchronous AM demodulation.
